1. Field of the Invention
This invention relates to a method for the real-time, non-destructive evaluation of welds, thermocompression bonds or the like by stress-wave emission techniques. In particular, a method is disclosed for distinguishing between mechanical noise and stress wave emission signals detected during welding, bonding or like operations.
2. Description of the Prior Art
The evaluation of adhesion bonds, welds or the like using real-time, non-destructive stress wave emission techniques is well known. For instance, U.S. Pat. No. 3,965,726 which issued on June 29, 1976 to S. J. Vahaviolos and is assigned to the instant assignee, describes the real-time evaluation of welds by monitoring emitted stress waves. Stress Wave Emissions (SWE's) may be defined as elastic waves which are characterized by low amplitude, short duration and fast rise time signals which are propagated in a structure as a result of an applied stress. As described in the aforementioned patent, the SWE energy emitted from the weld area during the solid-to-liquid phase transformation and the liquid-to-solid phase transformation are measured. The stress wave energy emitted during the liquid-to-solid phase transformation is subtracted from the stress wave energy emitted during the solid-to-liquid phase transformation and that value is compared with a predetermined value to determine the acceptability of the weld.
Another application relating to the use of SWE evaluation techniques is described in U.S. Pat. No. 4,004,456 which issued on Jan. 25, 1977 to S. J. Vahaviolos and is assigned to the instant assignee. In that patent, SWE energy is measured during the compression phase of an adhesion bond and that value is compared to a predetermined substantially linear relationship between the emitted stress wave energy and the strength of the particular adhesion bond being formed.
The foregoing techniques have been quite successful in determining the quality or the strength of bonds. However, at times, undesirable noise vibrations are caused by the head of the welding tool or bonder or the like. Such mechanical vibrations result in relatively high amplitude oscillating signals which last for an extended period of time and which are incorrectly interpreted by the stress wave detection equipment as SWE signals, causing incorrect evaluation and attendant loss of acceptable product.
Accordingly, there is a need for a method to distinguish between SWE signals and undesirable mechanical noise.